Alberto has weakened this morning to a depression in the Atlantic due to colder water temps and unfavorable upper level conditions. This system is no longer any threat to land and will likely dissipate as it moves northeastward out to sea.

In the Eastern Pacific Tropical Storm Bud has emerged and will likely turn northward towards the Mexican Coast. There is some considerable uncertainty as to exact timing of a landfall but it is possible this weekend. Bud may reach hurricane strength and interests along the MX Pacific Coast need to monitor this system this week.

As of this morning Alberto remains a tropical storm and is drifting slowly west-southwestward off the coast of South Carolina. At this time a direct landfall is not expected but some gusty winds and rain may impact the shore areas there. There is a tropical storm watch along the SC coast and it is possible some tropical storm conditions may reach the beaches later today or on Monday. Little change in strength is forecast during this time. Alberto is not forecast to make actual landfall and, after a southwestward drift today, it is likely going to turn northeastward and accelerate out to sea.

As of 5pm EDT on 5/19/2012 Tropical Storm Alberto has developed with max sustained winds of 45 mph. Forecast uncertainties remain due to weak steering flow in the upper atmosphere. This may warrant tropical storm watches tonight for the Carolinas as it is not far off shore and could threaten the coast. At this time some modest strengthening is possible.

The East Pacfic has come to life early in terms of tropical weather. Tropical Depression One-E has developed well south of the Mexican Coast several weeks before the official start of the 2012 Hurricane Season. It is no threat to land but it is an interesting development. It is too early to tell if this implies a busy year but none the less it seems we already off to the races!

Taking place now through early 2013, a major upgrade to the WSR-88D Doppler Radar network will be completed by the National Weather Service, promising significant improvements in near-term forecasting and warning technology across the United States.

The advanced technology is called dual-polarization (or dual-pol) radar. Up until now, conventional radar could only detect the horizontal dimension of particles in the air. Dual-pol radar will detect both horizontal and vertical dimensions and yield a much better estimate of the size, shape, and composition of precipitation particles. Armed with this new information, forecasters will be better equipped to obtain the following:

Better estimates of total precipitation
Better identification of precipitation type, rain, snow, sleet or mixtures
A more exact location of the rain/snow line
A more clearly defined hail core and updraft core in thunderstorms
Better detection of airborne debris associated with tornadoes
Easier determination of hazardous icing conditions for aircraft operations
Better identification of snow levels in higher terrain
Better ability to distinguish between precipitation and non-precipitation objects (ground clutter and biological targets)

Some limitations, as with current radar technology, will continue. In particular degradation in data quality due to distance from the radar antenna or storms lining up along a single radial from the antenna and “filling” the radar beam will continue. However, the benefits cited above are expected to vastly improve short term forecasting and warnings. For example, forecasters should have much better confidence in locating the transition line between rain and snow, estimating the amount of snowfall, determining the risk of flooding, distinguishing between heavy rain and hail, and confirming the presence of a tornado. With respect to the last item, dual-pol radar may allow the NWS to enhance tornado warnings with information signaling a more dire prediction of significant or catastrophic damage.

Only well after the national upgrade is complete will all of the benefits of this newly deployed technology be realized. It is seen as an exciting time for meteorologists and forecasters. For the general public it is also a very positive development with the expectation of improved warnings, and more reliable near term forecasts. Check with your local National Weather Service office for information on when you may see dual polarization in your area (or if it is available already).

Five of the major weather disasters in the U.S. last year involved outbreaks of strong and violent tornadoes. These predominantly occurred in the middle to southeast part of the country during April through June.

A review of weather data analyses at various levels of the atmosphere during that 3-month period showed a convergence of several features that are key to the development of supercell (rotating) thunderstorms that can produce tornadoes.

These key ingredients are described as follows (see figure below):

1.Warm, moist air in low levels from warmer than normal Gulf of Mexico waters

A stronger than normal flow of air within 5000 feet of the ground off the warmer than normal western Gulf of Mexico into the southern and central Mississippi River Valley provided warm, moist air that fueled and maintained large thunderstorms.

2.Hot, dry air in mid levels from drought stricken Texas

A stronger than normal flow of hot, dry air from the southwest launched off the drought-stricken Edwards Plateau of northwest Texas and southern plains lofted eastward at 5 to 10 thousand feet above the Mississippi River Valley added to the depth of instability and wind shear in the environment. Dry air at this level when cooled by rain helped to strengthen the rear flank downdrafts of supercell thunderstorms that are often key to the formation of tornadoes.

3.Cold air aloft

A persistent broad upper level low pressure trough over the northwestern US provided a source of colder air aloft and further increased the depth of instability in the atmosphere. Deep instability enhances vertical movement of air and strengthens thunderstorm updrafts and downdrafts.

4.Stronger than normal jet stream

A stronger and more persistent polar jet stream aloft from over the North Pacific Ocean across the southern and central Rockies into the middle US remained focused and locked in position by the broad upper level trough over the northwestern US and above normal high pressure ridging over northern Mexico.
A stronger than normal jet stream aloft causes rapidly developing low pressure systems or cyclones that induce thunderstorm development. This scenario is often associated with La Niña, cooler than normal equatorial waters in the Pacific.

5.Enhanced upward vertical motion in central US

A broad area of enhanced upward vertical motion over the central U.S. from the Great Lakes to the northern Gulf States helped maintain organized lines and clusters of thunderstorms. Enhanced upward vertical motion is dynamically induced in the exit region of a strong jet stream aloft.

The fact that these key ingredients are clearly depicted in analyses of weather data averaged over a 3-month period (not shown), April-June 2012, suggests that the dynamical features were predominant and repetitive. It also suggests persistent blocking in the hemispheric flow pattern kept upper level troughs and ridges locked in position for long periods of time. With a persistent blocking pattern, similar weather features (highs, lows, and frontal boundaries) can predominate or repeat over the same area for several weeks or even months leading to extremes in temperature, precipitation, and/or dryness.

What about 2012?

It is difficult to predict from one year to the next the amount and intensity of severe weather we are apt to experience across the U.S. Climatology suggests that the greatest threat of significant springtime tornadoes will continue to be centered in Oklahoma stretching eastward across the Deep South and northward into Iowa. And even if the configuration of circulation patterns change, there are many ways to bring key ingredients together to trigger supercell thunderstorms and the associated elevated risk of significant tornadoes.

That said, there are differences in the pattern this year from last year. The La Niña has weakened and is expected to transition to neutral conditions this spring. However, as the recent outbreaks of tornadoes in the middle US has shown us, the residual La Niña pattern may occasionally reappear with branches of the jet stream across the Pacific merging to produce accelerated flow and energy to disturbances crossing the continental U.S. (see figures below).

No matter what stands out to each of us about 2011, a common thread shared by many was the severity of nation’s weather. Varied, atypical, and at times both devastating and tragic, 2011 was an exceptionally active year for major-impact weather events in the US.

The National Oceanic & Atmospheric Administration identified no fewer than a dozen events with economical damage exceeding a billion dollars. The events are listed in chronological order below. How many of these “billion-dollar disasters do you recall?

Arguably, the most notorious weather events from 2011 were tornadoes. In fact, 2011 was a record breaking year for confirmed tornadoes in the US. Five spring-time severe storms events, totaling 827 tornadoes, resulted in nearly 500 fatalities with at least a billion dollars in losses caused by each event. Some of these tornadoes were long-track storms that were on the ground for prolonged periods. 84 tornadoes were rated as strong and violent, and was the second largest number of these potentially most destructive tornadoes that have occurred since 1950 (Fig.1).

Fig. 1. Tornadoes are rated on a scale (Enhanced Fujita scale) from 0-5. The potentially most destructive tornadoes (strong to voilent; EF3-EF5) from March-August in the US since 1950-2010 are shown above (countrsey NOAA). The 84 strong-voilent tornadoes in 2011 were the second highest total since 1950 and the most since 1974.

So, what were the factors that drove the exceptional 2011 tornado season? Are those factors discernable in the climatological data, and to what extent might they again occur in 2012? These are topics for our next discussion, coming in February 2012.

With the arrival of the New Year, many people east of the Mississippi River have been wondering what has happened to winter. Most areas of the Eastern US have seen well below normal snowfall and unseasonably warm weather. This is especially attention grabbing following the previous two winters, when much of the East saw well above normal snowfall and below normal temperatures. The reasons accounting for winter’s mildness thus far is the continuation of La Nina along with positive North Atlantic (NAO) and Arctic Oscillations (AO). These features drastically influence longer term global weather patterns and their state.

Most have heard of La Nina (cooler than normal equatorial waters in the Pacific), or it’s opposite El Nino (warmer than normal) and their influences on winter weather patterns (Fig.1); but unless you are in the field of meteorology, you are likely not aware of the NAO and AO, two principal mechanisms driving winter weather in the East.

The NAO is an oscillation of the pressure difference between an area of low pressure near Iceland and high pressure near the Azores in the subtropical Atlantic. When the NAO is positive, there is a significant difference in pressure between the two with a strong Azores High. When the NAO is negative, the difference in pressure weakens and the aforementioned Azores High is not as strong (Fig. 2). The AO is similar to the NAO, except it represents oscillations in atmospheric pressure in the polar region and the central Atlantic. The AO takes into account the difference between low pressure near the North Pole and high pressure over the central Atlantic. When the AO is positive, the pressure difference is greater with the central Atlantic High being strong. When the AO is negative, the opposite is true with a small difference and weaker area of high pressure in the mid latitudes (Fig. 3).

So, just how have the North Atlantic and the Arctic Oscillation led to a mild winter and a lack of snow in the Eastern US thus far? As previously stated, the positive phase of both oscillations results in strong high pressure in the mid and subtropical latitudes of the Atlantic. This is exactly what has been happening this winter with the western periphery of strong high pressure dominating the Eastern US. As a result, the higher pressures have kept the Arctic air from progressing southward. This has also resulted in a very unfavorable storm track for snowfall in the East with winter storms tracking well west and north of the Eastern US

It is lower pressures in the central Atlantic associated with a negative North Atlantic Oscillation and negative Arctic Oscillation that are more favorable for setting up weather patterns conducive for cold and snow in the Eastern US. So far the winter of 2011-2012 has yet to see this occur. With that in mind, the oscillation patterns could change as the current state of the science does not allow for an accurate prediction of the oscillation patterns more than a just few weeks in advance. Stay tuned.

Tropical Storm Sean grazed Bermuda but is now rapidly pulling away to the Northeast away from the island. The system with max sustained winds will likely encounter cooler waters and wind shear which will result in weakening. The storm will not likely be a threat to the US or Canada. Sean will most likely dissipate or be absorbed by a front well out at sea.

In another active season in terms of storm numbers, it is not surprising that November has yielded yet another system. In this case it is Sean which has transitioned from a subtropical cyclone to a tropical storm. In reality this classification is more academic than anything else, as the intensity has not changed, and a large area of tropical storm force winds still extends well north of the center. This system will likely remain in the Atlantic as it passes west of Bermuda than north and east out to sea. Only modest intensification if any is expected.